66 THORAX 



been noted also that the inferior mediastinum is separable 

 into three parts: (i) the anterior mediastinum, anterior to the 

 pericardium, (2) the posterior mediastinum, posterior to the 

 pericardium, and (3) the middle mediastinum, occupied by the 

 pericardium, the heart, the great vessels immediately adjacent 

 to the heart, and portions of the phrenic nerves, with their 

 accompanying vessels. These sections of the mediastinum 

 and their contents must now be examined in detail. 



Dissection. The remains of the superior and anterior 

 mediastinal parts of the pleura must be divided longitudinally, 

 immediately posterior to the sternum, from the lower end of the 

 thorax to the apices of the pleural sacs. The sternal extremities 

 of the first ribs must be then cut through, close to the manubrium 

 sterni, and, at the same time, the sternal heads of the sterno- 

 mastoid muscles must be separated from the manubrium, if 

 that has not already been done by the dissector of the head and 

 neck. After the sterno-mastoid muscles and the first ribs are 

 divided, the sterno-hyoid and sterno-thyreoid muscles must be 

 cut through transversely, as close to the upper margin of the 

 manubrium as possible. Next, the body of the sternum must 

 be separated from the xiphoid process and the tips of the seventh 

 costal cartilages. The sternum with the attached costal carti- 

 lages may then be removed and placed aside, but it must be 

 carefully preserved for future use. 



When the sternum is removed the mediastinum is exposed 

 from the front. As seen from the front, the superior medias- 

 tinum, which lies posterior to the manubrium, is a relatively 

 wide triangular area, with its apex below. The anterior medias- 

 tinum, on the other hand, is merely a narrow cleft between 

 the adjacent anterior margins of the pleural sacs, except opposite 

 the anterior end of the left fifth costal cartilage, where the left 

 pleural sac deviates slightly to the left and the anterior medias- 

 tinum becomes slightly wider (Fig. 27). 



The anterior parts of both the superior and the anterior 

 mediastina are occupied by areolar tissue in which, as far down 

 as the third or fourth costal cartilages, remains of the thymus 

 may be found. It may be recognised by its position and by the 

 relative firmness of its substance (Figs. 22, 23). 



Thymus. The thymus is a bilobed organ, developed 

 from the third visceral pouches. It is well developed in the 

 foetus and in the child until the end of the second year. 

 Then it frequently undergoes atrophy, but it may persist even 

 until old age. 



Dissection. Remove the thymus and the remains of the 

 mediastinal pleura and clean the anterior contents of the superior 

 mediastinum and the pericardium. Commence with the right 

 innominate vein and trace it from the upper aperture of the 

 thorax to its termination in the superior vena cava. It is joined 

 at its commencement, in the angle between the internal jugular 



